Nudibranch mating habits are weird… First, all nudibranchs are hermaphrodites (which means they are both male and female at the same time) – so when nudis mate, they both have the chance to get “pregnant”. I say chance because apparently some nudibranchs can decide whether they use the sperm they receive to fertilize eggs or if they’ll just consume the sperm and absorb it into their body. Some species of nudibranchs have a needle-like appendage that they stab their partner in the head with, injecting them with a mind-control substance which is thought to help increase the odds that it will use the sperm to fertilize eggs. Some other nudibranchs also have disposable penises that when they are done mating, they simply break it off and let their partner crawl away with it still in them. Like I said… weird stuff.
But I did manage to spot these two mating during one of my dives in Amed. Those white tubes connecting the two is the key. When I saw this during the dive, I definitely knew what it was, but the other divers in my group had moved away so I didn’t get to make sure they knew what it was. When I asked when we got to the surface if they noticed, none of them believed that I had been able to tell that for certain while in the water (as nudibranchs quite often trail each other like this, often nibbling on the tail of the nudi they are following)… until I showed them the photo.